Storm Event Effects on Water Quality
Storm Event Effects on Water Quality
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The dramatically increased flow during flood events affects many aspects of water quality. We can monitor the effects of storms on the watershed by comparing the responses of the characteristics to the depth of the water. Four key aspects observed during this event were specific conductance, turbidity, pH, and dissolved oxygen. |
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Culverts before storm event. |
Culverts during storm event of April 29, 2008. |
| Specific conductance, a measure of how easily water conducts electricity, adjusted for water temperature, showed a small increase at the very beginning of the storm event, followed by a sudden drop. The small increase indicates a "first flush", as readily available conductive materials, often deposited on surfaces such as roads, are immediately swept to the river by the earliest drops of rain. However, these early, accessible reserves of conductive materials were exhausted rapidly. Instead of bring more electrolytes down to the stream, further rain only diluted these conductive materials further. |  Specific Conductance of Nequasset Stream Below Hedge Bridge Road During Flood of april 29, 2008 |
 Turbidity of Nequasset Stream below Hedge Bridge Road during Flood of April 29, 2008 |
Turbidity (a measure of water clarity dependent on suspended particles) exhibits a large first flush. Similarly to specific conductance, the most readily available particles are quickly swept into the stream. With turbidity, however, the effect was far more dramatic, jumping from 3 to 100 NTUs. However, the turbidity peaked before the peak depth, indicating that the most readily available particles had already been swept into the stream. Further rain only diluted the particles, and by the time the depth peaked, the turbidity had already dropped to about 40 NTUs. In the middle of the decline is a small secondary peak in turbidity that is a result of a short increase in rainfall near the peak of the flood. As the storm event passed on down the stream, so did the turbid particles, resulting in an almost full recovery of pre-event levels. |
| The storm event caused the river to become more acidic. This makes sense, as various pollutants in the atmosphere make rain more acidic than water already on the ground. Therefore, rain should increase the concentration of acids in the stream, lowering the pH. The increase in acidity appears to mirror the increase in depth almost exactly, indicating that the newly arrived rain caused behaved in exactly this manner. The flood also suppressed the natural cycle of pH peaks that relate to biological activity in the stream. As the rain passed through the stream and depth began to return to normal, the pH also began to return to its original levels. |  pH of Nequasset Stream Below Hedge Bridge road during Flood of April 29, 2008 |
 Dissolved Oxygen in Nequasset Stream Below Hedge Bridge Road During Flood of April 29, 2008 |
The levels of dissolved oxygen in the stream decline at the beginning of the storm. As photosynthesis is a major source of oxygen in aquatic environments, this effect is most likely the result of the reduced sunlight due to cloud cover. Without sunlight, the process of photosynthesis was retarded becasue aerobic organisms consumed oxygen faster than it could be created. During the storm, however, dissolved oxygen increases due the more vigorous flow. After the storm, the presence of sunlight led the dissolved oxygen to recover and increase beyond the pre-storm value. |
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This graph shows the relationship between rainfall and the depth at Hedge Bridge Road. From this graph, we can see a significant lag between when the rain came down and when the stream increased in depth. This can be clearly observed in the time difference between peak rainfall and peak depth. This phenomenon occurs because all of the rain does not fall directly into the stream. Instead, rain falls throughout the watershed. Rain that falls in distant parts of the watershed can take a while to run down into the stream. Thus, the time lag reflects the average travel time for the stormflow runoff. |
Precipitation in Nequasset Stream Below Hedge Bridge Road During Flood of April 29, 2008 |
